1. Field of the Invention
The present invention relates to a wavelength selective optical switch device used at a node or the like in an optical communication system.
2. Discussion of the Related Art
A wavelength multiplexing optical communication technique is applied to a high-speed and large-capacity optical network that supports today's advanced information-telecommunication society. A ROADM (Reconfigurable Optical Add Drop Multiplexer) device having a reconfigurable add-drop function has been increasingly introduced to an optical node, which is a branch point in the optical network. To realize the ROADM device, a wavelength selective switch (also referred to as WSS) for switching light having a desired wavelength to a desired direction has received attention. A light beam deflection element for selecting wavelength and deflecting a light beam to a desired output port is used in the wavelength selective switch. U.S. Pat. No. 7,162,115B2 and U.S. Pat. No. 6,707,959B2 propose a light beam deflection element utilizing mechanical displacement of an MEMS (Micro-Electro-Mechanical System) mirror array, and US2006/0067611A1 and U.S. Pat. No. 7,397,980B2 propose a light beam deflection element utilizing a diffraction phenomenon of an LCOS (Liquid crystal on silicon) element.
In the optical communication field, in order to meet increasing demand for an increase in transmission capacity in recent years, a higher transmission rate and new modification format have been actively researched and developed, and an optical network has become complicated. In such optical network, the optically variable filter capable of changing a light beam having a desired wavelength in optical signal is used. In such optical filter, in order to achieve optimum filtering with respect to the transmission rate and modification format of each optical signal, there is a demand for a function of dynamically controlling a filter center frequency and passband at an optical frequency level in addition to the conventional frequency selective function.
This function can be realized by using the high-definition LCOS element, MEMS mirror array or the like for allotment of a plurality of pixels to each of channels of a WDM signal, that is, light having different wavelengths, and using a multi-level optical phased array for deflection of the light beam.
As described in US2006/0067611A1, “A Review of Phased Array Steering for Narrow-Band Electrooptical Systems” by Paul F. Mcnamonetal., Proceeding of the IEEE, Vol. 97, No. 6, pp. 1078-1096, June 2009, and “Optical Phased Array Technology” by Paul F. Mcnamon et al., Proceeding of the IEEE, Vol. 84, No. 2, pp. 268-298, February 1996., in the conventional multi-level optical phased array, a maximum phase shift amount in design wavelength has been generally set to 2 π.